Naturally found in the bone marrow, the stem cells have an awkward habit of spontaneously transforming into different cell types when stored outside the body.

The University of Glasgow team overcame this problem by creating ball-shaped clumps of around 10,000 stem cells, just big enough to see with the naked eye.

When they were placed in a soft gel with the consistency of bone marrow jelly, the cell balls remained unchanged for long periods of time and displayed no unwanted differentiated.

Placing the 3D cultures next to laboratory "wound models" simulating damaged cartilage, bone and ligament, the stem cells re-activated. They migrated towards the injured tissue and began to develop into the right kind of cells to promote healing.

Dr Catherine Berry, from the university's Institute of Molecular Cell and Systems Biology, said: "This is a really exciting discovery, which uses a fairly simple and affordable method to grow and maintain stem cells ready to heal tissues."

To make the stem cell bundles, the team used an ingenious technique that exploits magnetism.

Microscopic magnetic nanoparticles were first added to the stem cells, allowing them to be drawn together by the attractive force of a simple magnet.

The research, published in the journal ACS Nano, also has implications for the treatment of leukaemia and breast cancer.

Both diseases feature bundles of cancer stem cells that lie dormant in bone marrow for many years before re-activating and triggering disease recurrence.

Dr Berry added: "It may be that changes in mesenchymal stem cell activity are linked to cancer cell activation.

"We're keen to explore how we can use our technique to understand more about how stem cells communicate with other cells and what we can do to use stem cells more effectively in medicine."